Square and rectangular tube ASTM A500

ASTM A500 is a standard for cold-formed welded and seamless carbon steel structural tubing established by the American Society for Testing and Materials. It is widely used in structural engineering due to its high production efficiency, low cost, and effective guarantee of weld quality through high-frequency resistance welding.

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Details Introduction

ASTM A500 is the standard for cold-formed welded and seamless carbon steel structural tubing set by the American Society for Testing and Materials. Welded rectangular tubing (rectangular or square cross-section) is widely used in building structures, machinery manufacturing, and other fields due to its high strength and good formability. Its production process adds a "cold forming" key step on the basis of welded steel pipes. The core is to roll the round welded pipe into a square/rectangular shape through a mold. The specific process is as follows:

I. Raw Material Preparation

Steel Strip Selection
Select low-carbon steel or low-alloy steel hot-rolled steel strips that meet the chemical composition requirements of ASTM A500 (usually with a carbon content ≤0.25%), and the thickness is determined according to the wall thickness of the square/rectangular tube (commonly 1.5-12mm).

The steel strip needs to be surface cleaned to remove oxide scale, oil stains, rust, etc., to avoid pores or incomplete fusion defects during welding.
The steel strip is leveled and sheared into fixed-width dimensions. The width needs to be calculated and determined according to the perimeter of the square/rectangular tube and the forming process (leaving a welding margin).

Steel Strip Pretreatment

In some cases, the steel strip needs to be annealed to reduce hardness and increase plasticity, which is convenient for subsequent cold forming (especially thick-walled steel pipes).
Eliminate the wavy or sickle bend of the steel strip through a leveling machine to ensure uniform stress during forming.

II. Round Tube Blank Forming and Welding

This stage is similar to the ASTM A53 electric resistance welded steel pipe (Type E) process. The core is to first make round welded pipes. The specific steps are:

Continuous Roll Forming
The steel strip is gradually bent into a round tube blank (“rolling circle”) through multiple sets of horizontally and vertically arranged forming rollers. During the process, the rolling parameters (pressure, speed) need to be precisely controlled to ensure uniform roundness of the tube blank and tight fit at the joints (gap ≤0.1mm).

High-Frequency Resistance Welding (ERW)

The joints of the round tube blank are heated by high-frequency current (100-500kHz), and the heat generated by the metal resistance is used to bring the joints to a plastic state (approximately 1200-1300℃).
At the same time, pressure is applied through extrusion rollers to make the metal atoms at the joints diffuse and combine to form a solid weld (without filler material).
Immediately after welding, spray water to cool the weld to control the heat-affected zone (HAZ) range and avoid coarse grains affecting mechanical properties.

Preliminary Post-Welding Treatment

Remove burrs inside and outside the weld (using a special scraper or grinding wheel) to avoid scratching the mold or causing stress concentration during subsequent forming.
Perform online non-destructive testing (such as ultrasonic testing) on the welded pipe to identify internal weld defects (incomplete fusion, slag inclusion, etc.).

III. Cold Forming (Round to Square/Rectangular)

This is the core link in the production of square/rectangular tubes. The round tube blank is rolled into the target square/rectangular cross-section by cold rolling. The specific process is:

Multi-Pass Roll Forming
The round welded pipe enters a continuous cold bending unit and is progressively rolled through multiple sets of molds of different specifications (gradually transitioning from round to square/rectangular):

First pass: Press the round tube into a “racetrack shape” (straight on both sides, arc-shaped on the top and bottom) to initially change the cross-sectional shape.
Subsequent passes: Gradually adjust the mold angle and spacing to roll the tube blank into an approximate square/rectangular shape, focusing on controlling the arc of the four corners (the fillet radius needs to meet the ASTM A500 requirements for “corners”, usually 1.5-3 times the wall thickness).
During the forming process, it is necessary to strictly control the rolling speed and pressure to avoid wrinkles, cracks, or dimensional deviations (such as side length and diagonal tolerance) in the tube body.

Sizing and Straightening
The formed square/rectangular tube is further corrected for dimensional accuracy (such as side length, perpendicularity, and straightness) through a sizing unit:

Straightness requirement: usually ≤1.5mm bending per meter, ≤0.15% bending of the total length.
Cross-section size tolerance: side length deviation ≤±0.75mm (small size) or ±1% (large size), diagonal difference ≤1.5mm.

IV. Heat Treatment (Optional)

According to product requirements, some ASTM A500 square/rectangular tubes need to be heat treated to eliminate the internal stress generated by cold forming and improve mechanical properties:

Stress Relief Annealing : Heat the steel pipe to 590-650℃, keep it warm for a period of time and then slowly cool it to reduce internal stress (especially thick-walled or high-strength steel pipes).
After heat treatment, the mechanical properties (tensile strength, yield strength, elongation) need to be re-tested to ensure that they meet the requirements of Grade A/B/C in ASTM A500 (such as Grade B yield strength ≥345MPa, tensile strength 435-550MPa).

V. Finishing

Fixed-Length Cutting
Cut according to the length required by the customer (such as 6 meters, 12 meters), using sawing or plasma cutting to ensure that the incision is flat, burr-free, and the end face perpendicularity deviation is ≤1°.

Surface Treatment

Black pipe (uncoated): Remove surface oxide scale by shot blasting or pickling to enhance subsequent coating adhesion.
If corrosion protection is required, treatments such as painting and galvanizing (hot-dip galvanizing or electro-galvanizing) can be performed. The thickness of the galvanized layer is usually ≥85μm (in accordance with ASTM A123 standard).

End Processing (Optional)
In some applications, the pipe end needs to be beveled (such as for welding connections) or threaded (such as for threaded connections).

VI. Quality Inspection

Dimensional Inspection
Measure side length, wall thickness, diagonal difference, fillet radius, etc. to ensure compliance with the dimensional tolerances of ASTM A500.

Mechanical Property Testing
Sampling for tensile testing (measuring yield strength, tensile strength, elongation), flattening test (verifying weld plasticity, flattening to 1/3 of wall thickness without cracks in the weld).

Non-destructive Testing

Welds: Ultrasonic testing or eddy current testing to identify internal defects.
Surface: Visual inspection, free from defects such as cracks, dents, overlaps, etc.

Hydrostatic Test
Apply test pressure to the steel pipe (usually ≥5MPa), hold pressure for more than 30 seconds, and pass if there is no leakage (testing weld sealing).

Summary of Core Features

The production process of ASTM A500 welded square and rectangular tube is based on “round tube welding → cold forming”. The cold forming process achieves cross-sectional conversion through multiple passes of roll forming, which not only ensures the dimensional accuracy of the square and rectangular tube, but also improves the strength of the steel through cold work hardening. Compared with seamless square and rectangular tubes, it has high production efficiency, low cost, and the weld quality is effectively guaranteed by high-frequency resistance welding, so it is widely used in structural engineering.

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